➜ ~ composer diagnose
Checking platform settings: WARNING
The OpenSSL library (0.9.8za) used by PHP does not support TLSv1.2 or TLSv1.1.
If possible you should upgrade OpenSSL to version 1.0.1 or above.
But this doesn't appear to match the version of OpenSSL I've actually got running.
➜ ~ which openssl
/usr/local/opt/openssl/bin/openssl
➜ ~ openssl version
OpenSSL 1.0.2k 26 Jan 2017
I realise there are similar questions but as the most popular answer suggests using the now broken command
brew link --force openssl
which as per the comments doesn't work.